Microelectromechanical accelerometers are currently being manufactured for a number of applications including vehicle airbag and inertial navigation and guidance systems. For applications such as vehicle airbags the accelerometers need to be both accurate and inexpensive.
Microelectromechanical accelerometers are formed on a wafer using fabrication process steps similar or identical to those used in integrated circuit fabrication. Microelectromechanical devices combine electrical and mechanical functionality into one device. The fabrication of microelectromechanical devices is generally based on the making and processing of alternate layer of polycrystalline silicon (polysilicon) and a sacrificial material such as silicon dioxide (SiO2) or a silicate glass. The polysilicon layers are built up and patterned layer by layer to form the structure of the device. Once the structure is completed the sacrificial material is removed by etching to release the polysilicon members of the microelectromechanical device for operation. The removal of sacrificial material in some microelectromechanical accelerometers includes using an isotropic release etch to release beams of the accelerometer from the bottom surface of the accelerometer. This release etch has the disadvantage of etching away part of the beams and reducing the proof mass and effectiveness of the accelerometer.
The type of accelerometer fabrication described above provides an accelerometer that is co-planar with the plane of the wafer. Using this method two accelerometers can be fabricated in one wafer to measure acceleration in two orthogonal directions, both co-planar with the plane of the wafer. A different accelerometer design is required for an accelerometer to measure acceleration perpendicular to the plane of the wafer.